1. Field of the Invention
This invention generally relates to downhole well tools and specifically to controlled buoyancy perforating methods and apparatus.
2. Description of Related Art
Traditional petroleum drilling and production technology often includes procedures for perforating the wall of a production well bore into the fluid bearing strata to enhance a flow of formation fluid along perforation channels. Depending on the well completion equipment and method, it is necessary for such perforations to pierce a wellbore casing, a production pipe or a tube wall. In many cases, the casing or tube is secured to the formation structure by a cement sheath. In such cases, the cement sheath must also be pierced by the perforation channel as well.
There are three basic methods presently available to the industry for perforating wells. Those three methods are: a) explosive propelled projectiles, b) pressurized chemicals and c) shaped charge explosives. Generally, however, most wells are perforated with shaped charge explosives. Accordingly, the preferred embodiment description of the present invention will be directed to shaped charge perforators. However, many of the invention characteristics may be adapted to other perforation methods.
Shaped charge explosives are typically prepared for well perforation by securing a multiplicity of shaped charge units within the wall of a steel pipe section. The pipe section bearing the shaped charges may be supported from the wellhead at the end of a wireline, coiled tube, coupled pipe or drill string for location within the wellbore adjacent to the formation zone that is to be perforated by detonation of the shaped charges.
Collectively, a pipe section and the associated charge units will be characterized herein as a “charge carrier.” One or more charge carriers may be coupled serially, end-to-end, to provide a unitized gun section. A “perforating gun” may include one or more gun sections that are joined by swivel joints. A perforation gun is merely one of many “bottom-hole assemblies” or bottom-hole tools the present invention is relevant to.
Each shaped charge unit in a charge carrier comprises a relatively small quantity of high energy explosive. Traditionally, this shaped charge unit is formed about an axis of revolution within a heavy steel case. One axial end of the shaped charge unit is concavely configured. The concave end-face of the charge is usually clad with a thin metallic liner. When detonated, the explosive energy of the decomposing charge is focused upon the metallic liner. The resulting pressure on the liner compressively transforms it into a high speed jet stream of liner material that ejects from the case substantially along the charge axis of revolution. This jet stream penetrates the well casing, the cement sheath and into the production formation.
A multiplicity of shaped charge units is usually distributed along the length of each charge carrier. Typically, the shaped charge units are oriented within the charge carrier to discharge along an axis that is radial of the carrier longitudinal axis. The distribution pattern of shaped charge units along the charge carrier length for a vertical well completion is typically helical. However, horizontal well completions may require a narrowly oriented perforation plane wherein all shaped charge units within a carrier section are oriented to discharge in substantially the same direction such as straight up, straight down or along some specific lateral plane in between. In these cases, selected sections of charge carriers that collectively comprise a perforation gun may be joined by swivel joints that permit individual rotation of a respective section about the longitudinal axis. Additionally, each charge carrier may be asymmetrically weighted, for example, to orient a predetermined rotational alignment when the gun system is horizontally positioned.
Controlled Buoyancy Perforating (CBP) allows the use of long perforating gun sections in horizontal and extended reach wells by reducing the weight and increasing the buoyancy of the perforating equipment. Reduction of the gun weight correspondingly reduces the bearing weight of the gun against the horizontal segments of the borehole wall and hence, the frictional forces opposing axial movement of the gun string along the well bore length. CBP objectives are accomplished by a combination of designs and materials such as composite material carrier tubes, caseless perforation charges and foamed material charge holders. Other inventions and innovations that pertain to Controlled Buoyancy Perforating (CBP) are described in U.S. patent application Ser. No. 10/696,697 which is incorporated herein by reference.
Although the thrust of CBP is focused upon reductions of the gun weight, the requirements of internal seal integrity from an external fluid pressure environment and rapid assembly and disassembly on the rig floor remain the same as known to the prior art. Also imperative of CBP is a rig floor assembly system that confidently maintains a predetermined angular orientation of the perforation charges.
Prior art perforating guns are, generally, a serial assembly of charge carriers, end-to-end, in 30 ft. to 90 ft. segments. As the longitudinal axis of a charge carrier segment is suspended vertically from a derrick crown block, the lower end of the segment is aligned with the upper end of a tool string or preceding charge carrier segment that is suspended vertically within the well bore from the rig floor; usually by a slip accessory in the rotary drive table. A threaded end connector joins the adjacent ends of the axially aligned segments when either segment is rotated relative to the other about the longitudinal axis common to both.
Although threaded steel carrier connections as previously described are suitably strong for supporting the enormous weight of a steel perforating gun, the incremental assembly process is relatively slow. CBP technology greatly alleviates these joint loads on a gun assembly. Where a 5 in. conventional steel perforating gun may weigh in excess of 14 lb/ft., a similar, CBP composite material system may weigh only 4 lb/ft. A 5,000 ft. long perforating gun having a weight distribution of only 4 lb/ft. requires the upper end connectors to support a 20,000 lb air weight load. As a CBP gun is lowered into the well and the gun weight is supported by the displacement forces of the wellbore fluid, the tensile loads on the connectors and connector threads is negligible. However, after the gun is discharged, the gun buoyancy is dramatically reduced by the consequential flooding of the internal gun volume. Hence, even though CBP technology may reduce the stress demands on a charge carrier connection, significant strength requirements remain.
One of the driving objectives of CBP, therefore, is to place extremely long perforating guns in substantially horizontal production bores. Reduction or elimination of the rotational steps in the charge carrier assembly process could greatly accelerate the perforating gun assembly procedure.
It is an objective of this invention, therefore, to provide a bayonet joint connection between charge carrier joints that requires no rotation.
Another objective of this invention is a rapidly assembled bayonet connection between charge carrier joints that maintains a predetermined angular orientation between the joints.
Also an object of this invention is a steel connecting collar between non-metallic housing tubes for charge carrier joints.
A still further object of this invention is a method and apparatus for rapid preassembly of an inner loading tube within an outer carrier housing that requires no intermediate booster assembly.